1. Field of the Invention
The present invention relates to an organic electroluminescence device (hereinafter also referred to as organic EL device) using an electroluminescent organic compound emitting light by injection of an electric current and provided with an emission layer made from such an organic EL material and to a process for fabricating the same.
2. Description of the Related Art
Typically, an organic EL device using an organic compound material is an electric current injection type element having diode characteristics, as shown in FIG. 1, and the element emits light with luminance dependent on an amount of electric current. A display panel having a matrix arrangement of a plurality of such elements has been being developed (Japanese Unexamined Patent Publication No. Hei 8-315981). The display panel uses a substrate fabricated in the following manner. An indium-tin-oxide (so-called ITO) film is deposited on a glass substrate 2 serving as a monitor screen, and the obtained film is patterned by means of etching, thereby to obtain a substrate with an anode 3a thereon that is a transparent electrode. Each organic EL device 20 constituting the display panel has a multi-layer structure wherein a plurality of organic compound material layers 4 including a light-emitting layer and a cathode 5a (a metal electrode) have been deposited in this order on the anode 3a (a transparent electrode) by vapor deposition or the like. When required, the organic compound material layer 4 is provided with a hole transport function layer (a hole-injection layer, a hole-transport layer) and an electron-transport-function layer (an electron injection layer, an electron-transport layer) in addition to the light-emitting layer.
Conventionally, because the organic compound material layer is of high resistance, the layer having a minimized thickness has been formed in order to obtain a reduced driving voltage.
When the organic EL device 20 having the above-described constitution is fabricated by forming the organic compound material layer having a minimized thickness, there arises a problem caused by the existence of a dust of foreign matter in the vapor depositing process. As shown in FIG. 2, if a dust particle 6 exists, for example, on the anode 3a, in the vapor depositing process wherein the organic compound material layer 4 and the cathode 5a (the metal electrode layer) are successively formed on the glass substrate 2, the growing rate of the organic compound material layer 4 around the contact point between the dust particle and the anode 3a is reduced. This is because the particles obtained by vapor deposition are hardly deposited on the periphery of the part hidden by the dust particle 6 (the hidden part 6a). Accordingly, the film thickness of the organic compound material layer 4 around the dust particle 6 i.e., the film thickness of the organic compound material layer 4 between the anode 3a and the cathode 5a tends to be thinner compared with that of the other part. Consequently, the luminance may be locally changed because of current concentration caused by the proximity of the anode 3a and the cathode 5a around the dust particle 6. The element 20 could even be broken by occurrence of a short circuit caused by contact between the anode 3a and the cathode 5a. 
Accordingly, a countermeasure such as the cleaning of the substrate 2 prior to deposition of each layer is taken, but the complete removal of the dust which has stuck is hardly possible by mere cleaning.
In view of the problems described above, the object of the present invention is to provide an organic EL device in which an occurrence of current leak between the transparent electrode and the metal electrode is reduced and a process for fabricating the element.
According to a first aspect of the present invention, there is provided an organic electroluminescence device which comprises a light-transmitting substrate; and
a transparent electrode, organic compound material layers including a light-emitting layer and a metal electrode which are formed in this order on the light-transmitting substrate;
wherein the organic compound material layers includes a light-emitting inter face of the light-emitting layer demarcating the organic compound material layers and the transparent electrode into a transparent electrode side portion and a metal electrode side portion;
wherein the transparent electrode side portion of the organic compound material layers and the transparent electrode has an interface with a largest difference in refractive indexes thereof; and
wherein the transparent electrode side portion of the organic compound material layer has a film thickness equal to or larger than a thickness corresponding to a minimum value between first and second maximum values of light-emission efficiency in luminescence efficiency characteristics depending on the film thickness of the transparent electrode side portion of the organic compound material layers.
In an aspect of the invention of the organic electroluminescence device, the light-emitting interface of the light-emitting layer emitting light having a wavelength of xcex serving as a primary component, and wherein the transparent electrode side portion in the organic compound material layers is deposited to have a film thickness such that an optical distance from the light-emitting interface to the interface with the largest difference in refractive indexes is substantially equal to even multiples of one-quarter of the wavelength xcex.
In another aspect of the invention of the organic electroluminescence device, the metal electrode side portion in the organic compound material layers is deposited to have a film thickness such that an optical distance from the light-emitting interface to an interface bordering on the metal electrode is substantially equal to odd multiples of one quarter of the wavelength xcex.
A second aspect of the present invention is to provide a process for fabricating an organic electroluminescence device including a light-transmitting substrate; and a transparent electrode, organic compound material layers including a light-emitting layer emitting primarily light with a wavelength of xcex and a metal electrode which are formed in this order on the light-transmitting substrate; wherein the organic compound material layers includes a light-emitting interface of the light-emitting layer demarcating the organic compound material layers and the transparent electrode into a transparent electrode side portion and a metal electrode side portion; and wherein the transparent electrode side portion of the organic compound material layers and the transparent electrode has an interface with a largest difference in refractive indexes thereof;
the process comprising the steps of:
providing a light-transmitting substrate on which a transparent electrode is formed;
layering organic compound material layers in turn on the transparent electrode to form a transparent electrode side portion of organic compound material layers so as to have a film thickness such that an optical distance from the light-emitting interface to the interface with the largest difference in the refractive indexes is substantially equal to even multiples of one quarter of the wavelength xcex;
layering a light-emitting layer and other organic compound material layers in turn on the transparent electrode side portion to form a metal electrode side portion of organic compound material layer so as to have a film thickness such that an optical distance from the light-emitting interface to an interface bordering on the metal electrode is substantially equal to odd multiples of one quarter of the wavelength xcex; and
layering a metal electrode on the metal electrode side portion of the organic compound material layer.
In an aspect of the invention of the process for fabricating an organic electroluminescence device, the organic compound material layers and the metal electrode are deposited by means of vapor deposition.
According to the present invention, using the phenomenon wherein the high order peaks in the light-emission efficiency makes an appearance with an increase in the film thickness of the organic compound material layer, particularly in the film thickness of the transparent electrode side portion of the layer, an organic EL device is provided wherein the prevention of current leak can be accomplished without impairing the light-emission efficiency (luminance with respect to electric current) by control of the film thickness of the organic compound material layer positioned between the transparent electrode and the light-emitting layer.